Corrosion behaviour of Inconel 625 in different application conditions
Inconel 625 is immune to intergranular corrosion by precipitation of niobium carbides at 927oC to 1038oC annealing temperature. Niobium carbides interact with carbon to precipitate as chromium carbides in the grain boundaries. Chromium depletion near the grain boundaries is resulted by intergranular chromium carbide precipitation. It occurs at specific temperatures and is called as sensitization. Alloy’s sensitization makes it less sensitive to intergranular corrosion. Inconel 625 can be made sensitive to this corrosion by an improper annealing processing that would prevent the development of niobium carbides hence making carbon more available to precipitate intergtranularly similar to chromium carbides by a subsequent sensitizing processing.
Integranular corrosion of alloy 625 due to sensitization of the grain boundaries is observed by annealing the alloy specimens at different temperatures and then subjecting them to sensitizing temperatures in the range of 704oC to 871oC for one hour. The specimens were then evaluated in boiling 65% nitric acid permitting the subsequent precipitation of chromium carbides at the grain regions when Inconel 625 is subjected to sensitizing temperatures up to 871oC. At lower annealing points, Niobium carbides are precipitated hence sensitizing heat processing of 704oC to 871oC will result into nominal or no precipitation of chromium carbides at the grain regions.
Performance in Bleach plant environment
Pulp and paper bleach plants involve the exposure of materials in solutions of pH 1.4 to 9.5 with 5500pm chlorides at 80oC and strong chlorine base oxidizers present. It is found that Inconel 625 is one of the best highly corrosion resistant alloys in bleach plant conditions. High chromium and molybdenum content of Inconel 625 result into its pitting resistance in this environment and pitting resistance in general.
Sea water corrosion
Different alloys have been classified by order of corrosion resistance to marine conditions. Inconel 625 is the best in marine environments owing again to its high chromium and molybdenum concentration. Inconel 625 shows supreme corrosion-fatigue strength in sea water.
Aqueous corrosion
However Inconel 625 is popular for its aqueous corrosion resistance, performance of Inconel 625 in high temperature oxidation conditions is good. A cyclic oxidation test is often conducted on high temperature alloys like Inconel 601, Inconel 617 and MA 956 to determine their scale adherence. The test was conducted at 982oC in which the alloy specimens were heated for 15 minutes and quenched in air for 5 minutes. Their weights were measured after each set of 10 cycles to determine the mass change. The samples were evaluated for 40 days. It is found that Inconel 625 offers the supreme resistance to cyclic oxidation showing its potential t retain a security oxide coating under the extreme cyclic condition.
Chlorine
Chlorine is a used as a key material in multiple industrial processes for example mineral chlorination, vinyl chloride production, fuel element processing and heat recovery systems. Inconel 625 shows the best corrosion resistance at the high temperatures such as 593oC to 927oC preventing liquid phase corrosion.